This invention relates to a scrubbing process and apparatus for separating hydrogen and carbon dioxide from a gas containing H.sub.2, CO.sub.2 and H.sub.2 S, said process being generally conducted at low temperature.
Conventionally, crude oils, refinery residual oils, or carbon constitute raw materials for the production of hydrogen-enriched gaseous mixtures (containing in addition thereto a greater or less amount of carbon oxides) which are utilized as starting mixtures for hydrogenations, ammonia synthesis, and methanol synthesis. Ammonia can in turn be reacted with CO.sub.2 to form urea. These raw materials which contain sulfur in most cases are subjected to an oxidative thermal cracking step with oxygen at an elevated temperature. After the separation of interfering components, such as carbon black tar, naphthalenes, higher hydrocarbons, and water, a gaseous mixture is then obtained consisting essentially of hydrogen, carbon monoxide, carbon dioxide, hydrogen sulfide, and traces of nitrogen, argon, and methane. If it is intended to form from this gas a starting gas mixture for ammonia synthesis or for hydrogenation hydrogen, then the carbon monoxide present in the gas is subjected to a conversion with steam, resulting in the oxidation of the carbon monoxide to carbon dioxide and the formation of additional hydrogen.
Before being employed for a synthesis, however, any such gas must be freed from the so-called "sour gas," inasmuch as the components constituting the sour gas, namely CO.sub.2, H.sub.2 S, COS, and mercaptans, would otherwise poison the synthesis catalysts. In case of a gas wherein the carbon monoxide was subjected to a conversion to carbon dioxide, there are no COS and mercaptans present since these are reduced to hydrogen sulfide under the conditions of the conversion; accordingly, in this case, the sour gas consists essentially of CO.sub.2 and H.sub.2 S.
To remove these sour gas components from the gaseous mixturs, so-called physical scrubbing methods have been used extensively for many years. In physical scrubbing, the sour gas components are absorbed without chemical binding and can be desorbed by expansion, heating and/or distillation. Polar organic solvents and among these particularly methanol, which can dissolve considerable amounts of sour gas at temperatures of below 10.degree. C., are especially suitable for this purpose. In this connection, it proved to be particularly advantageous that the Henry's law constants of H.sub.2 S and CO.sub.2 are different so that during a physical scrubbing step the differing solubilities of H.sub.2 S and CO.sub.2 in the scrubbing medium can be utilized for the selective scrubbing out of H.sub.2 S and CO.sub.2.
Such a process is disclosed, for example, in "Linde Reports on Science and Technology" No. 18 of 1973, pages 7 to 13 incorporated by reference herein. In this conventional process, a gas consisting essentially of H.sub.2, CO.sub.2, and H.sub.2 S is scrubbed with methanol in a scrubbing column having three stages, the gas flowing through this column from the bottom toward the top. In the lowermost stage, which is the H.sub.2 S scrubbing stage, the H.sub.2 S is washed out with a minor quantity of methanol loaded with CO.sub.2 withdrawn from the scrubbing stage located thereabove. This step has the advantage that only a very small amount of CO.sub.2 is dissolved from the raw gas at this point; consequently, the heat of solution is relatively minor. In the second scrubbing stage arranged thereabove, the primary amount of the CO.sub.2 is dissolved in partially loaded methanol. The residual CO.sub.2 is washed out in the uppermost section of the scrubbing column which is subjected to a spray of very cold methanol, thereby removing the substantially last traces of sour gas components from the gas, down to the range of 10 p.p.m. The methanol utilized in the top of the column is obtained by thermal regeneration.
In the conventional method, the scrubbing media of the first and second stages are expanded twice, respectively, thus liberating dissolved gases, and are then fed at different levels to an H.sub.2 S enrichment column where the CO.sub.2 still present in the scrubbing media is stripped out with nitrogen, and simultaneously the H.sub.2 S liberated during the stripping step is reabsorbed, so that a residual gas is discharged at the head of this column consisting essentially of nitrogen and carbon dioxide. Further processing operation provides a thermal regeneration of the scrubbing media to obtain an H.sub.2 S fraction and a methanol-water separation, after which the scrubbing agent is again available in regenerated form.
This conventional method works well, but for the production of urea, further difficult steps would be required to form pure CO.sub.2 from the exhaust gas leaving the H.sub.2 S enrichment column. Specifically, for urea production, up to approximately 75% of the carbon dioxide contained in the raw gas must be obtained in the pure form, if the hydrogen purified in such a system is scrubbed, e.g., to high purity in a subsequent liquid nitrogen washing stage and fed, after adding further nitrogen, to the NH.sub.3 synthesis as a 3H.sub.2 + N.sub.2 mixture ready for the synthesis, the CO.sub.2 being then reacted to urea with the thus-produced ammonia.